1. Field of the Invention
This invention relates to semiconductor wafer processing and more particularly to a wafer which can sense and record processing conditions to which the wafer is exposed, and can also write the recorded processing conditions to an external output device brought in electrical contact with the wafer.
2. Background of the Relevant Art
The fabrication of an integrated circuit generally employs numerous processing steps. Once a starting material or bulk substrate is provided, many masking layers and processing steps are presented to the substrate to form an overall semiconductor topography. The topography includes diffusion regions, dielectrics, contacts, metallization and passivation necessary to form an integrated circuit.
An exemplary sequence of steps involves growing thin film material upon the wafer substrate. Thereafter, photoresist is coated upon the thin film and a lithography mask is imaged upon the photoresist in order to allow radiation to polymerize certain photoresist areas. The non-polymerized photoresist can be removed and previously deposited, underlying thin film material (e.g., polycrystalline silicon, metallization, silicon oxide, silicon nitride or spin-on glass) can be etched away to form a desired geometric structure. Individual masks requiring many processing steps are needed in order to form active areas into which, for example, field effect transistors (FETs) are lithographically placed. Capacitor plates and/or dielectrics as well as resistive elements can also be formed upon the substrate topography in order to assist FET operation.
Each process step must be carefully monitored in order to provide an operational integrated circuit. Throughout the imaging process, deposition and growth process, etching and masking process, etc., it is critical, for example, that temperature, gas flow, vacuum pressure, chemical gas composition and exposure distance be carefully controlled during each step. Careful attention to the various processing conditions involved in each step is a requirement of optimal semiconductor processing. Any deviation from optimal processing conditions may cause the ensuing integrated circuit to perform at a substandard level or, worse yet, fail completely.
Conventional techniques used for monitoring processing conditions generally involve various transducers placed within the processing chamber or upon the chamber wall. The transducers attempt to read the processing conditions to which the wafer is exposed. However, in many chambers, there is a significant distance between the wafer and the transducer location. If, for example, the transducer is placed on the deposition or anneal chamber inner wall, the transducer will read a different temperature than the temperature to which the actual wafer is exposed. It is well known that, for example, temperature, gas flow rate and/or gas composition is dissimilar at the chamber wall as opposed to the middle of the chamber, where the wafer generally resides. The thermal conductivity of a wafer is not equal to the thermal conductivity of the ambient chamber area or chamber wall. Still further, areas of laminar and non-laminar flow exist throughout the chamber. While transducers on the chamber wall may indicate laminar flow within a specified flow rate, the wafer placed near the center of the chamber may instead be subject to deleterious non-laminar flow outside acceptable flow specification limits. There exists many further examples of processing condition readings taken from the chamber wall or ambient within the chamber which do not correspond to readings at the wafer surface. In order to precisely determine processing conditions at the wafer, it is critical that measurements be taken upon the wafer and the readings be recorded in situ.
As defined herein, "processing conditions" refer to various processing parameters used in manufacturing an integrated circuit. Processing conditions include any parameter used to control semiconductor manufacture and/or semiconductor operation such as temperature, processing chamber pressure, gas flow rate within the chamber, and gaseous chemical composition within the chamber. Processing conditions still further include parameters used to measure vibration and acceleration (or movement) of the wafer through the chamber, and to control the accurate placement of an image and etchant upon the wafer. Specifically, it is important to monitor the relevant position of a mask with respect to the wafer surface. Many types of exposure techniques are used such as contact printing, proximity printing, projection printing and step-and-repeat printing. Each of these exposure techniques many require an accurate determination of the distance between the wafer and the mask as well as the distance between the wafer and the radiation source. Conventional exposure techniques may use monitors placed on the printing equipment which mechanically measure the distance between a wafer chuck or holder and the mask or source. In order to determine a true distance between the wafer's upper surface and the mask or source, approximation may be needed as to the wafer's thickness. Generally, a standardized thickness is used. Unfortunately, wafers usually have varying thickness depending upon the number of processing steps used or upon the initial substrate thickness. Dissimilar wafer thickness can lead to inaccurate or imprecise knowledge as to the relative distance between the wafer's upper surface and the mask or source.
Wafer thickness not only determines proper mask or source placement, but it also determines wafer etch effectiveness. A standard dry etch chamber utilizes a chamber filled with a gas (generally reactive gaseous material) and a pair of electrodes, wherein one electrode is sized to accommodate a wafer. Many conventional dry etch chambers having spaced electrodes cannot achieve optimal etch of the wafer unless the spacing between one electrode and a wafer placed on the other electrode is calibrated prior to and monitored throughout each wafer etch operation. Slight changes in the gap or distance between the wafer and the opposing electrode may substantially change the plasma etch rate. It is well known that etch rate varies depending upon the spacing between the wafer and the opposing electrode as well as the operating pressure, temperature and gas flow rate exerted upon the wafer. Etch rate often increases as the voltage across the electrodes (sheath voltage) increases. Furthermore, sheath voltage will increase as the spacing or gap decreases, or if the rf voltage upon a powered electrode increases. Conventional methods used to calibrate and monitor spacing generally use mechanical means such as clay balls to measure and calibrate the electrode gap. Clay balls deform when opposing electrodes contact the balls, a resulting measurement can then be achieved from the deformed balls. Measurements taken from the balls do not accurately and consistently correspond with the gap which causes the deformation. Thus, indirect, mechanical measurement generally lacks accuracy needed for true calibration.
In order to overcome the problems with mechanical calibration, a more precise measurement/calibration technique using non-mechanical (i.e., optical) measurement principles has been recently devised, and is described in co-pending, commonly owned, U.S. patent application Ser. No. 08/033,025 (herein incorporated by reference). Patent application Ser. No. 08/033,025, illustrates optical linear encoders mounted to the chamber housing instead of directly upon the wafer itself. Although linear encoders provide suitable calibration between the electrodes, they are not directly coupled to the wafers, nor can they directly read the actual distance between one electrode and a wafer surface of a wafer arranged on the other electrode. Again, approximations are needed to extrapolate the actual distance based upon the measured distance taken at the housing-mounted linear encoder.
It is important not only to measure processing conditions upon the wafer, but it is equally important to measure processing conditions across the entire wafer surface. Oftentimes, processing parameters vary across the wafer surface. Gas flow chambers generally place a greater flow rate near the center of the chamber than at the wafer edges (i.e., near the chamber walls). As such, large area wafers placed in such a chamber may receive greater gas nucleation and/or deposition near the wafer center than at the edges. Further, due to the radiant heat of the chamber walls, a greater temperature exists at the edge of the wafer than at the center. These are but a few examples of processing condition gradients which occur across a standard wafer. Modern wafers of eight inch diameter or larger are even more susceptible to processing condition gradients. In order to reduce the deleterious effects of such gradients, it is important to first ascertain that gradients occur, and then to closely monitor their occurrence during actual wafer processing. Unless the gradients can be sensed at various points across the entire wafer surface, the gradients cannot be ascertained and certainly cannot be monitored in situ.